1. Field of the Invention
The invention relates to an apparatus providing a means for inserting a workstring into an earth borehole. With additional particularity, the invention relates to apparatus for passage of well tools from the surface of the earth into an earth borehole which is pressurized or may become pressurized during operations being conducted with the well tools. With greater particularity, the invention relates to a means for passage of full sized tools, that is, tools with outer diameters as large as the inner diameter of the borehole, into a pressurized or potentially pressurized earth borehole, without loss of pressure control, without the use of either the blind or annular blow-out preventers to prevent loss of pressure control, and without loss of redundant blow-out preventer protection in the event of an emergency condition. Such apparatus have particular, but not exclusive, utility in the fields of oil and gas exploration and production where highly pressurized flammable fluids may be encountered in the wellbore during various operations.
2. Description of the Prior Art
In the practice of drilling earth boreholes, subsurface pressures are most frequently controlled by the hydrostatic pressure of the wellbore fluid column, typically comprising drilling mud or completion fluid. As long as the hydrostatic pressure created by such wellbore fluid column is at least as great as the subsurface formation pressure, no formation fluid may flow into the borehole. However, situations arise where the subsurface formation pressure is greater than the hydrostatic fluid column pressure. This may be unintentional, where unexpectedly high subsurface pressures are encountered during well operations, or intentional, where an oil or gas zone to be produced is intentionally perforated in an "underbalanced" mode in order to prevent wellbore fluids, containing solid particulates, from flowing into the oil and gas zone and potentially damaging the ability of said zone to produce minerals. In other drilling, completing, or workover operations on an oil or gas well, it is often necessary to introduce workstrings (such as wireline, coiled tubing, or snubbing strings) conveying various tools (such as logging tools, perforation tools, nitrogen injection tools, etc.) into the wellbore under pressure, or at least when pressure conditions present a substantial risk of occurring. In situations where wellbore fluid pressure is insufficient to control fluid flow from a pressure bearing subsurface formation, alternative means must be relied on to control the subsurface pressure; the safety of the rig personnel, rig equipment, and the rig itself depend on this pressure control. The ability to control subsurface pressure is also critical to prevent the loss of valuable natural resources, such as oil and natural gas, and to prevent environmental pollution resulting from an uncontrolled fluid flow.
During normal drilling operations the weight of the wellbore fluid is typically adjusted ("weighted up") to produce wellbore hydrostatic fluid pressure that is higher than any formation pressure encountered, therefore the only surface equipment used to control wellbore pressure is a "blow-out preventer" assembly. Typically two different types of independently operable blow-out preventers are included in series, to provide a redundant safety system. One type of preventer typically included in a blow-out preventer assembly is an annular type which is capable of closing off the wellbore while no workstring is in the well; in addition, the annular preventer will close and seal around a workstring in the wellbore, without damaging the workstring. Another type of blow-out preventer typically included in a blow-out preventer assembly is a "blind" or "blind/shear" type, capable of completely closing the wellbore at the surface while no workstring is in the wellbore, or if a workstring is in the wellbore, sealing the well while crushing and/or shearing any pipe, wireline, or tubing that may be in the wellbore. Blind or blind/shear blow-out preventers are not capable of sealing a pressurized wellbore while at the same time allowing pipe, wireline, or tubing to be passed into the well, and annular blow-out preventers are subject to damage if used in this mode.
Rather, apparatus used to allow sliding passage of pipe, wireline, or tubing while simultaneously maintaining pressure seal of a wellbore are typically referred to as "pressure lubricators". While no representation is made thereto, the name of such equipment presumably emanates from the fact that some form of lubrication is typically applied at the sliding interface of the seal and wire or tubular being passed therethrough.
One conventional pressure lubricator configuration, such as disclosed in U.S. Pat. No. 4,836,289 to Young, includes an extended tubular with a flared lower end. The flared end of said tubular is lowered into the wellbore and the annular blowout preventer closed about the outer diameter of said tubular, to both hold it in place and to provide a lower pressure seal. A lock pin mandrel is installed atop the blowout preventer assembly, and lock pins in that mandrel are advanced into a mating profile on the tubular to further secure the tubular in place. The lock pins and the flared tubular end, when contacting the annular preventer element, prevent the tubular from being forced out of the well due to pressure from below. The annular blowout preventer necessarily must remain closed about this type of lubricator during operations therethrough. Disadvantages to this conventional design are several: the outer diameter of the tubular must be smaller than that of the inner diameter of the annular blowout preventers, thereby limiting the size of tools that may be inserted into the well, and use of the annular blow-out preventers to provide a lower lubricator pressure seal effectively disables that blow-out preventer from preventing escape of pressure at the top of the lubricator assembly, forcing reliance in such event solely on the blind or blind/shear rams, use of which will damage pipe, wireline or tubing extending therethrough when the blind rams are closed. Accordingly, by using the annular blow-out preventer to provide a pressure seal about the lubricator, not only is redundance of critical safety equipment lost, but use of the safety equipment which remains is likely to cause property damage. In addition, rig personnel must climb up on the blowout preventer and wellhead assembly or on scaffolding to manually access and advance the lock pins. This procedure is inherently unsafe.
Another type of lubricator assembly is disclosed in U.S. Pat. No. 4,553,591 to Mitchell, Nov. 19, 1985. In Mitchell a dual flanged spool is mounted between the annular blowout preventer and the bell nipple, a cylindrical wear bushing having an axial bore secured therein by radial pins, and a mandrel having an internal diameter less than the blow-out preventer bore diameter connected to the axial bore of the cylindrical bushing. The size of wireline tools that may be used with this mandrel in place is restricted, since Mitchell teaches that the internal diameter of the mandrel is less than the blowout preventer bore diameter in order to prevent "whipping" of pipe, wireline or tubing in the blowout preventers. Further, multiple lockdown pins must be manually advanced in the annular spool to engage the mandrel and cylindrical wear bushing. To advance the pins, one or more drilling rig personnel typically climb upon the blow-out preventer stack or make-shift boards and scaffolding placed around same, which may be at a great distance above ground or water level and too often presents a difficult, slippery, muddy surface to climb upon.
Neither of the aforesaid patents provide apparatus and method for providing a wellbore pressure seal which provide the advantages of:
i) having a full-bore inner diameter, at least as large as the wellbore diameter, so as to permit maximum workstring tool size passage therethrough; PA1 ii) having a pressure seal at the lower end of the pressure control apparatus independent of the annular or ram-type blowout preventer, thereby retaining redundant blow-out prevention capability; and PA1 iii) being able to be installed entirely from the rig floor without removing existing equipment, thereby saving time and avoiding the necessity of rig personnel to climb upon and about the blowout preventer stack to manually advance lockdown screws or the like. PA1 1. a blowout preventer assembly comprising a blind and/or blind/shear ram and an annular blow-out preventer superimposed thereon; PA1 2. a single flanged seal body having internal concentrically disposed axial threads and seal surfaces, superimposed on the annular blow-out preventer; PA1 3. a bell nipple sealingly engaged with the seal body; PA1 4. a tubular mandrel sealingly engaged with the inner threads and seals of the seal body, the mandrel having one or more standpipe sections extending upwardly therefrom; PA1 5. a protector for the inner threads and seal surfaces of the single flanged seal body; PA1 6. a tool for running and retrieving said protector; PA1 7. means for forming a pressure seal between the inner diameter of said mandrel or standpipe and a workstring slidably disposed therein, superimposed at the top of the mandrel/standpipe assembly.
The invention disclosed herein represents a significant improvement over existing lubricator stack assemblies in that all of the aforesaid advantages, in combination, are embodied therein in an economically deployed, safe apparatus having few components.